1. Field of the Invention
The present invention relates to an insulating coupler or an insulating amplifier (hereinafter, referred to as an isolator) for transmitting an electric signal using a capacitor formed on a semiconductor device and particularly a highly dielectric capacitor which does not break down the device and prevents a dangerous voltage from passing the secondary side even if a high voltage is applied.
2. Description of Related Art
To protect a highly public network equipment and a terminal, the communication field requires high isolation for the boundary between the network and the terminal and a miniature communication transformer having high isolation has been used. However, as personal terminals have been spread and developed, further miniaturization and lightweight have been required as a portable terminal, and a problem arises that a miniaturization request is not sufficiently satisfied by improvement of the material and structure used for a transformer, and application of an isolator has been examined.
In use such as measurement or medical treatment, there is a case that it is necessary to isolate the signal detection unit and the signal processing unit from each other such as a sensor and signal processing circuit and an isolator is known as an isolation means in such a case.
As an object of miniaturization, high reliability, and low price of an isolator, a capacitive isolator has been developed. As a highly dielectric capacitor art as an individual part constituting an insulating barrier, a ceramic capacitor for power or surge protection is known and a signal transmission circuit block using it is called a capacitive insulating amplifier or a capacitive insulating coupler and has been used since 1970.
In Japanese Patent Application Laid-Open 62-260408, a pulse signal modulated from an analog signal is transmitted to the secondary side isolated by a capacitor, and an analog signal is regenerated by decoding, and isolation of transmission of the analog signal is realized.
The aforementioned prior art is an excellent method which can make the capacity of the isolation capacitor smaller such as 3 pF because it modulates an analog signal at a higher frequency. However, it is necessary to further consider prevention of occurrence of an error due to electrical noise when a signal is to be transmitted via the isolation capacitor. The prior art is easily affected by electrical noise in essence because it analogously modulates an analog signal such as PWM (pulse width modulation) or FM (frequency modulation) and transmits it. The system described in Japanese Patent Application 62-260408 has a possibility of transmission of not only an analog signal but also a digital signal which is mostly used in the recent signal processing. Therefore, transmission of a digital signal in which noise can be easily removed by the signal processing is specified and a method for preventing a transmission error due to electrical noise will be examined.
In the case of transmission of an analog signal, the effect of electrical noise appears as an error of the analog value proportional to the intensity of electrical noise. In transmission of a digital signal, the effect will not appear at all before the intensity of electrical noise exceeds a threshold value. However, when the intensity of noise exceeds a threshold value, the transmission is affected. When each bit has an individual meaning, even if any bit is incorrectly transmitted due to electrical noise, it is greatly affected. In the case of data whose analog value is converted to a digital value, the bits on the MSB (most significant bit) side are generally affected more greatly. As mentioned above, especially in transmission of a digital signal, when electrical noise more than a threshold value is generated, the effect is suddenly increased and cannot be ignored in practical use. Particularly isolation is often used for long distance signal transmission or in a device installed out of doors in which a lightning surge is expected, so that the frequency that the intensity of electrical noise due to the induced surge becomes higher than the level affecting transmission of a digital signal is high.
To eliminate the effect of electrical noise during signal transmission, redundant codes having a long Hamming distance between codes such as error correcting codes or CRCs (cyclic redundancy code) have been conventionally used widely. To make correction of a 1-bit error possible, it is necessary to set the Hamming distance between the redundant codes to 3 or more. There is a tendency that the decoding process such as redundancy coding and error correction becomes complicated.
An object of the present invention is to provide an information transmission system via an isolating capacitor having little error effect due to electrical noise.
Another object of the present invention is to provide a transmission system using redundant codes which can correct an error at a smaller Hamming distance and can be simply realized.
The first and second objects are accomplished by the following constitution or method.
Firstly, a signal transmission apparatus comprises a redundancy coder for redundancy-coding an input signal, a decoder for decoding a signal redundancy-coded by the redundancy coder, and an isolator for electrically insulating the redundancy coder and decoder and also transmitting information from the redundancy coder to the decoder, and the primary side (the transmission source side from the isolator) redundancy-codes a signal (error detection and correction code) and transmits it via the isolator, and the secondary side (the transmission destination side from the isolator) detects and corrects an error during decoding. By doing this, a signal is redundancy-coded (error detection and correction code) and transmitted. Therefore, by redundancy-coding (error detection and correction code) and transmitting, an error is detected and corrected during decoding, so that a transmission error due to electrical noise can be eliminated.
Secondarily, when the code transmitted during decoding by the decoder on the secondary side is a non-coded word (a code incoherent as a redundant code), the transmission apparatus is constituted so that the already decoded signal, that is, the preceding output value is held. By doing this, by a redundant code at a smaller Hamming distance, signal transmission with little error effect is made possible. Namely, when the transmitted code is a non-coded word, by holding the preceding output value, by the redundant code at the Hamming distance 2 between the codes, pseudo-error correction is made possible. When an error occurs at the change point of a signal to be transmitted, a delay occurs in the change in the transmitted signal by one interval of signal transmission. However, if the signal transmission interval is shortened, the delay in signal transmission when an error occurs can be minimized.
Thirdly, a signal transmission apparatus comprises a modulator for modulating an input signal in synchronization with the clock signal, a demodulator for regenerating the input signal by synchronizing the signal modulated by the modulator with the clock, and an isolator for electrically insulating the modulator and demodulator and transmitting information from the modulator to the demodulator. By doing this, the modulated input signal is transmitted via the isolator and demodulated, so that electrical noise can be eliminated. Furthermore, when the clock signal inputted to the demodulator is sent to the modulator via the isolator and the input signal is modulated in synchronization with the clock signal, the effect of electrical noise on demodulation of the modulated signal can be extensively reduced without being affected by electrical noise. Therefore, the effect of synchronous detection having the effect of electrical noise elimination can be produced at its maximum.
Fourthly, a signal transmission apparatus comprises a modulator for modulating an input signal in synchronization with the clock signal, a demodulator for sampling a signal obtained by modulating the signal modulated by the modulator at the leading edge and trailing edge of the clock, and an isolator for electrically insulating the modulator and demodulator and transmitting the modulated signal from the modulator to the demodulator. By doing this, modulation and demodulation can be realized by a digital process, so that a circuit performing a sure operation can be easily obtained. Furthermore, when the clock signal inputted to the demodulator is sent to the modulator via the isolator and the input signal is modulated in synchronization with the clock signal, the effect of electrical noise on demodulation of the modulated signal can be extensively reduced without being affected by electrical noise.
Fifthly, a signal transmission apparatus comprises a signal output device for outputting a signal in synchronization with the clock signal, a signal receiving device for receiving the signal from the signal output device by the clock signal only for a specified period, and an isolator for electrically insulating the signal output device and signal receiving device and also transmitting a signal from the signal output device to the signal receiving device, and the primary side outputs a signal to be transmitted in synchronization with the clock signal, and the secondary side operates the receiving function only at the timing that the transmitted signal is transferred in synchronization with the clock signal. According to this, the function on the secondary side for receiving the transmitted signal operates only at the timing that the transmitted signal is transferred in synchronization with the clock, so that the effect of electrical noise occurring at random without correlation with the clock can be eliminated.
Furthermore, when a modem is constituted using the signal transmission apparatuses shown in the first to fifth examples, a modem for eliminating the effect of electrical noise can be provided.
Furthermore, when the signal transmission apparatuses shown in the first to fifth examples are applied to an information processor such as a personal computer, an information processor having a built-in modem eliminating the effect of external electrical noise can be provided and when a signal outputted from the signal transmission apparatus is processed by a microprocessor which is a personal computer, the personal computer can be miniaturized.